Summer used to be network television's dead zone, stocked with repeats and cheap reality shows, and it was a place to bury canceled series.

That left an opening for the cable networks, which saw a chance to gain viewers without having to go up against the networks' best new shows in the fall, using summer to launch hit programs such as "Mad Men."

You can't close down for the summer ... There's opportunity year-round.— Jennifer Salke, NBC Entertainment President

CBS fired back last summer with the Stephen King miniseries "Under the Dome," which did so much better than expected that the network is bringing it back this year with fresh episodes. That success, combined with increasing competition from Netflix, Amazon and other upstart content providers, has upended traditional notions of what constitutes a TV "season."

"You can't close down for the summer," said NBC Entertainment President Jennifer Salke. "There's opportunity year-round."

All the TV sizzle, however, carries significant risks. Networks are shelling out big money for these summer dramas, with Fox's summertime reboot of "24" with Kiefer Sutherland topping $4 million per episode in total production costs, according to industry insiders. "Extant" costs nearly that much. That matches the tab for the most expensive shows during the regular season.

At the same time, the cable networks are ramping up their own offerings to stay competitive. In addition to FX's marquee summer project, TNT is rolling out the apocalyptic epic "The Last Ship." The show's executive producer is Michael Bay, best known for directing big-budget summer films including the "Transformers" series.

"Our goal is to make watching TNT be like going to a great summer movie," Michael Wright, the programming chief of TNT's parent company, Turner Networks, told media buyers earlier this month in New York. "Grab a bucket of popcorn, kick back and let us take you on a thrilling ride."

In an interview, Bay said he had wanted to get involved in TV for a long time but had been "slow and cautious" about making his move. "This particular idea was batted around quite a bit, and it was really cinematic," he said of "The Last Ship." "So I gave it my all."

Cable networks have long realized, of course, that summer offers a perfect opportunity to chase new viewers. HBO, for example, moved its vampire drama "True Blood" from fall to summer starting with Season 2 back in 2009. Viewing more than doubled, according to Nielsen, and it became one of HBO's top-performing shows. Its seventh and final season will air this summer.

Those kinds of opportunities existed for cable networks because for decades broadcasters mostly ignored the summer. With longer days and summer vacation competing for viewers' attention, the networks scheduled mostly repeats between Memorial Day and late September, when the new season started. Over the last decade, they found success with relatively inexpensive reality fare, such as "America's Got Talent,""Big Brother" and "So You Think You Can Dance."

Much of the credit for the about-face goes to a surprise hit from last year. "Under the Dome" was adapted from a Stephen King thriller about townsfolk trapped under a giant transparent bubble.

The show succeeded despite a number of traditional hindrances to attracting big audiences. In addition to the summer time slot, there were no high-profile stars, and it wasn't a cop show or medical drama that viewers would instantly recognize.

The series logged an average weekly viewership of nearly 14 million — an impressive figure that prompted CBS to order a second season to a program initially conceived for a single season. "Dome's" next season rolls out June 30.

But it was a clever financial deal that enabled CBS to greenlight the show in the first place. "Dome" was modeled after formulas that cable networks had exploited: limited runs (typically just 10 or 12 episodes per season), an intriguing concept and the type of marketing drumbeat usually reserved for grander events.

"Cable did teach us some lessons: that people are interested in the kind of bigger show that broadcast dramas can do well," said Neal Baer, a former writer-producer for "ER" and "Law & Order: Special Victims Unit" who's now behind "Under the Dome."

In an unusual arrangement brokered by talent agency WME, CBS hedged its risks by sharing costs with Amazon — exactly the sort of new-media company that usually gives network executives panic attacks because it threatens to encroach on their existing business.

Each episode costs about $3 million to produce, with CBS essentially splitting about half of the bill with Amazon. Foreign sales roughly pay for the other half. In exchange, Amazon won the right to deliver "Under the Dome" to viewers on its website within days of each episode's broadcast, far earlier than the weeks- or months-long delay that would typically exist.

"It was a unique deal that others have copied," NBC's Salke said.

Broadcasters are hoping to hedge their bets by finding production partners to split the costs. NBC initially tapped Georgeville Television, an independent production company, to help finance "Crossbones," although Georgeville later dropped out of the project. (NBC declined to comment.)

The series is a sort of "Apocalypse Now" meets "Pirates of the Caribbean," with Malkovich as a deeply menacing sixtysomething Blackbeard (the real pirate is believed to have died in his 30s). The writer-producer is Neil Cross, who won acclaim for his work on the BBC's "Luther" and "Doctor Who."

Of course, TV is a cyclical business. Today's reality-show breakthrough can easily turn into tomorrow's also-ran. But the creators behind TV series are convinced that something has changed. Viewers, they say, now demand — and expect — new content year-round.

And to stand out among the thick cloud of summer entertainment options, the networks courted movie talent. That's why CBS is putting a lot of promotional muscle behind "Extant," with Berry as an astronaut whose life takes a sinister twist after she returns from space. She is one of the very few Oscar winners for lead actress to sign up for a TV series (Kathy Bates starred in NBC's legal drama "Harry's Law," and Jessica Lange costars in FX's "American Horror Story").

Typically, film stars are wary of the minimum four-year commitment required for the traditional TV series. But "Extant" will air for just 13 episodes, giving it the kind of special-event feel in vogue for summer TV these days.

As "Extant" executive producer Greg Walker put it: "Summer's the time we all go to see event movies."

Now "that's what people want to watch on their television, as opposed to the old fare of reality shows or repeats," Walker said. "People don't want that anymore."

For the Record, 4:04 p.m.: This story about summer TV shows mischaracterized the deal-making behind CBS' drama "Under the Dome." While the talent agency WME was involved in the packaging of the licensing deal with CBS, the network itself brokered the streaming deal with Amazon.